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This year's ozone hole is shaping up to be one of the largest ever, having already surpassed the size of last year's.

The World Meteorological Organization (WMO) says that despite a relatively late start, the ozone hole has grown rapidly during the past couple of weeks and has now passed the maximum size attained in 2007.

According to the WMO, the hole in the ozone layer currently covers an area of 27 million square kilometres, compared to a maximum of 25 million square kilometres last year.

But the WMO says that while the hole continues to grow, it is still too early to determine how large it will be, before it breaks apart in mid-December.

Dr Paul Fraser, an atmospheric scientist with Australia's CSIRO, says that while this year's ozone hole is large, it isn't unexpected.

"Last year's hole was relatively small," says Fraser. "Most of the holes over the past few years have been large."

"This year's hole is shaping up to be a big one like 2006, which was one of the biggest."

Driven by weather

Ozone-depleting chemicals, such as chlorofluorocarbons (CFCs), were phased out under the 1987 Montreal Protocol, but continue to linger in the atmosphere.

Fraser says these chemicals are currently decreasing at a rate of approximately 1% per year.

Despite this, the size of the ozone hole is still largely determined by the stratospheric weather.

"The size is driven largely by the temperature and shape of the polar vortex - the air mass in the stratosphere in which the ozone hole forms," says Fraser.

"When it's exceptionally cold and symmetrical then you'll have a very deep ozone hole."

"We're not likely to see statistically significant changes in the ozone hole driven by those small changes [in ozone-depleting chemicals] for five to ten years."

Experts predict that the ozone layer will fully recover sometime in the second half of this century, but this is complicated by other atmospheric processes, such as an increase in the level of greenhouse gases.

"Not only do we have to predict the supply of chemicals, but we also have to predict the stratospheric temperatures," says Fraser.

"In the stratosphere, greenhouse gases cause cooling. So in the polar regions they enhance the ability of chemicals to destroy ozone, while the accumulation of greenhouse gases will hasten ozone recovery around mid-latitudes.

"So the role of greenhouse gases in ozone depletion is quite complicated."